With an explosive increase in the demand for a wideband multimedia communication service of the Internet, video distribution and the like, a long distant and highly reliable high density wavelength multiplexing optical fiber communication system has been introduced in a trunk line system or a metro system.
In such a large scale communication network, the reduction of operational cost is important as well as the reduction of capital cost. In order to reduce the operational cost, it is needed to optimize dynamic transmission capacity allocation and path switching corresponding to demands, efficient maintenance, a redundant configuration and the like. That is, it is desired to be able to flexibly reconfigure a communication network, and is also desired to reduce cost of reconfiguration.
In order to flexibly reconfigure the communication network at a low cost, an optical switch node capable of switching the path of light without converting a light signal into an electrical signal can be applied. Since the flexibility of the communication network depends on the degree of freedom of the path switching in the optical switch node, it is desired to provide an optical switch with a large number of ports.
In the optical switch, it is further needed to suppress the loss of signal light passing through the optical switch, wavelength dependence, polarization dependence, crosstalk to a non-connection path, and the like. Furthermore, suppression of power consumption needed for holding and switching of a signal light path, and miniaturization of an optical switch node are also important. These are issues directly related to the expandability of the number of ports of the optical switch, and the reduction in size, low loss, and low power characteristics are needed for the optical switch.
On the other hand, it is needed to suppress the occurrence of a problem related to communication service quality such as interruption of a line during communication, and to provide a stable communication state. Furthermore, a high speed path switching characteristic is also needed in order to prevent a line interruption state due to path switching from being recognized by a user. It is desired that such high speed switching can be performed in a small scale circuit with low power as much as possible.
As such an optical switch, there are an MEMS (Micro Electro Mechanical Systems) matrix optical switch, a liquid crystal matrix optical switch, a waveguide matrix optical switch and the like. The MEMS matrix optical switch is an optical switch that switches an optical path of spatial propagation beams by electrostatically controlling the direction of fine movable mirrors formed on a silicon substrate. The liquid crystal matrix optical switch is an optical switch that controls transmittance of spatial propagation beams by electrically changing the alignment state of liquid crystal molecules. The waveguide matrix optical switch is an optical switch that selects a signal light output path by changing an interference state by using temperature dependence of a refractive index of an optical material constituting a waveguide type optical interferometer.
The MEMS matrix optical switch is advantageous in terms of expandability of the number of input/output ports because insertion loss and signal light crosstalk are small. However, since about milliseconds are needed for a response of the movable mirror, the MEMS matrix optical switch is not suitable for use purposes needing high speed and high frequent path switching such as an uninterrupted optical path switching and optical packet signal switching. Furthermore, in the MEMS matrix optical switch, in the case of creating one input multi-output connection state, it is necessary to adjust a beam divergence angle in response to a distance between signal light output ports by sacrificing optical characteristics.
The liquid crystal matrix optical switch has a response time constant equal to that of the MEMS matrix optical switch. The liquid crystal matrix optical switch has high mechanical reliability because it has no movable part such as MEMS. However, in the liquid crystal matrix optical switch, since optical characteristics of liquid crystal molecules largely depend on temperature, degeneration and deterioration may occur in a high temperature environment. Accordingly, in the liquid crystal matrix optical switch, it is necessary to consider resistance to environment and reliability when it is actually applied to a system.
The waveguide matrix optical switch changes an interference state of a large number of waveguide optical interferometers formed on a substrate by electric field application, current injection, temperature and the like, thereby selecting an optical signal output path. The waveguide matrix optical switch is suitable for miniaturization as compared with the aforementioned two types of optical switch elements. Particularly, a waveguide matrix optical switch using silica is advantageous in terms of low loss because coupling efficiency with a single mode optical fiber is high. Furthermore, the waveguide matrix optical switch using silica has high mass productivity and reliability.
The basis configuration of the waveguide matrix optical switch is an optical gate switch that thermally controls interference conditions of the waveguide optical interferometers by applying temperature dependence (a thermo-optic effect) of a refractive index of an optical waveguide material. A time (about milliseconds) approximately equal to that of the MEMS matrix optical switch is required in order such that heat transfer between a core and a clad/a substrate reaches equilibrium. Accordingly, in the waveguide matrix optical switch, in order to shorten the time needed for reaching heat equilibrium, it is desired to make a volume (a sectional area of an optical waveguide) of a heated area in the optical gate switch as small as possible.
Under such situations, much attention has been attracted on an optical switch element using a semiconductor-based optical waveguide capable of significantly miniaturizing an optical waveguide element. As one of the technologies related to the optical switch element using the semiconductor-based optical waveguide, there is a silicon photonics technology. In the silicon photonics technology, a silicon film (SOI: Silicon On Insulator) formed on an insulating film (a film obtaining by thermally oxidizing a silicon substrate surface) is processed into a stripe shape and allowed to serve as an optical waveguide core. Since a refractive index difference between a core layer (a refractive index: about 3.48 @1550 nm) including the SOI film and a clad layer (a refractive index: about 1.45 @1550 nm) including a dielectric film (SiO2 and the like) is large, signal light is strongly confined into the core layer. Accordingly, when the silicon photonics technology is applied, it is possible to suppress insertion loss within a practical range even in a steep curve equal to or less than a curvature of 100 μm. Furthermore, in the silicon photonics technology, it is possible to expect high precision processing and mass productivity based on an advanced CMOS (Complementary Metal Oxide Semiconductor) process technology, and moreover, it is also possible to expect monolithic integration and the like with a driving circuit.
In the case of implementing an M input N output (hereinafter, M×N: M and N are natural numbers equal to or more than 2) matrix optical switch by using the silicon photonics technology, a M×N waveguide optical switch is arranged on two-dimensional lattice points on a semiconductor substrate and optical waveguide groups for connecting them lengthwise and breadthwise are arranged on the same semiconductor substrate. In this case, the optical waveguide groups mutually intersect on the semiconductor substrate.
Signal light inputted to the matrix optical switch is scattered when passing through an intersection region, so that loss occurs. The degree of the scattering correlates with a ratio of the size of a section of the optical waveguide and the size (a mode diameter) of a basic propagation mode of the signal light. Herein, in an optical waveguide using the silicon photonics technology, in which the refractive index difference between the core layer and the clad layer is high and optical confinement is strong, the influence of the scattering of the signal light in the intersection region appears notably. On the other hand, in an optical waveguide based on silica and the like in which the refractive index difference between the core and the clad is small and optical confinement is weak (a weak guided wave: weakly guiding), since the signal light shows a behavior approximate to a plane wave, it is less scattered when passing through the intersection region.
In this regard, in order to reduce the influence of the scattering of the signal light in the intersection region, it has been proposed that a matrix optical switch using the silicon photonics technology is configured using a rib type optical waveguide formed by thickening a part of a two-dimensional thin slab optical waveguide.
An electromagnetic field of a basic propagation mode of the rib type optical waveguide shows a sectional distribution of being approximately confined at inside (inside a silicon layer) than a refractive index boundary surface between a rib sidewall and the clad layer. Since it is possible to increase an effective sectional area by 1 digit as compared with a silicon wire, it is possible to allow signal light to propagate along the rib without taking into consideration of the refractive index boundary surface between the rib sidewall and the clad layer. In this case, it is possible to reduce the scattering of the signal light in the intersection region of the optical waveguide. A rib type optical waveguide-based optical switch element to which the silicon photonics technology is applied, for example, is disclosed in PTLs 1 and 2 and the like. Furthermore, there has also been reported a development example of a matrix optical switch using a 2 input 2 output (2×2) MMI (Multi-Mode Interference) optical multiplexer/demultiplexer based on the rib type optical waveguide to which the silicon photonics technology is applied. Moreover, a technology related to the optical waveguide is disclosed in PTL 3.